1. Field of the Invention
The present invention relates to a heater activating apparatus for activating a heater such as a halogen heater and the like, and more particularly, it relates to a heater activating apparatus for activating a heater so that a heating element heated by the heater is maintained at a predetermined temperature.
2. Related Background Art
Recently, in an image forming apparatus such as an optical printer copying machine, a heat fixing device wherein a toner image is heated or heated and pressurized by a heating element such as a heat roller which surface is maintained at a predetermined temperature has widely been used.
An example of a heater activating circuit which is used with such heat fixing device and which provides a background of the present invention is shown in FIG. 7.
In FIG. 7, the reference numeral 1 denotes an AC voltage source; 2 denotes a switch; 3 denotes a halogen heater; 4 denotes a large electric power thermistor (referred to as "power thermistor" hereinafter); 5 denotes zero cross-controlled SSR (solid state relay); 6 denotes a thermistor for detecting a temperature of the heater; 7 denotes a comparator having an open collector output; 8 denotes a transistor; and 9 through 13 denote resistors.
Next, an operation of the above mentioned heater activating circuit will be explained.
When the switch 2 is turned ON, the voltage V.sub.3 becomes a GND level (i.e., earthed), the transistor 8 goes to an OFF condition, and a light emitting element 5b of the SSR 5 also goes to an OFF condition. Since a triac 5a of the SSR 5 is in an OFF condition when the light emitting element 5b is turned OFF, the voltage is not applied to the heater.
On the other hand, when the switch 2 is turned OFF, the voltage V.sub.1 is determined by the partial voltage of the resistor 9 and the temperature detecting thermistor 6. That is to say, when the temperature of the thermistor 6 is decreased the value of V.sub.1 is reduced, and, when the temperature of the thermistor 6 is increased the value of V.sub.1 is also increased. If the voltage V.sub.1 is lower than a voltage V.sub.2 of the other input terminal of the comparator 7, the output V.sub.3 of the comparator 7 will be HIGH. In this case, since the transistor 8 is turned ON, the light emitting element 5b is also turned ON. Since the SSR 5 is zero cross-controlled, after the light emitting element 5b has been turned ON, the triac 5a is turned ON when the voltage at both ends thereof become zero or thereabout. And, the triac 5a is maintained in the ON condition until the voltage of the power source becomes zero or thereabout (i.e., the current in the heater becomes zero) after the light emitting element 5b has been turned OFF. When the temperature of the thermistor 6 increases to increase the voltage V.sub.1 higher than the voltage V.sub.2, the comparator 7 is inverted to provide a LOW output. Consequently, the transistor 8 is turned OFF, thereby deenergizing the heater 3. Such condition is shown in FIG. 10.
In FIG. 10, A indicates a time point when the temperature adjustment is initiated by turning the switch 2 OFF in a condition that the temperatures of the heater 3, power thermistor 4 and the temperature detecting thermistor 6 are reduced to a room temperature; and B indicates a time point when the heater 3 is energized in a condition that the heater 3 is controlled substantially at the set temperature and the temperature of the temperature detecting thermistor 6 is substantially in the set value. The heater current in the time point A is shown in FIG. 8, and the heater current in the time point B is shown in FIG. 9. The power thermistor 4 is used to control the rush current to the heater. The power thermistor has a resistance of a few ohms when the temperature of the power thermistor itself is cooled to the room temperature. However, when the power thermistor is heated by the heater current, the resistance thereof is reduced below one ohm, thereby reducing the power consumption.
In the above-mentioned arrangement wherein the power thermistor 4 is connected to the heater 3 in series, since the current is always flowing in the power thermistor while the heater is being activated, the temperature of the power thermistor is considerably increased. Since the power thermistor has the thermal time constant larger than that of the heater, once the power thermistor has been heated up, it is not readily cooled. Accordingly, such arrangement provides the rush current controlling action when it is cooled to the room temperature (see FIG. 8). However, upon normal temperature adjustment, since the temperature of the heater is decreased to some extent but the temperature of the power thermistor itself is till high, the resistance thereof is small as mentioned above, with the result that the rush current control cannot be attained (see FIG. 9).